The present invention relates to upper internal equipments for nuclear reactors and is particularly suitable for use in water cooled and moderated nuclear reactors.
It relates more particularly to internal equipments located over the core of a nuclear reactor, having a lower plate formed with openings for the coolant leaving the core, an upper plate, support columns connecting the plates together, cluster guides for control cluster vertically movable between a position in which they are inserted in the core and a position in which they are out of the core, each of the guides having an upper section fixed to the upper plate and a lower section extending between the upper and lower plates.
The purpose of the control cluster guides is to guide the rods of the cluster, to avoid deformation thereof and to protect these rods whatever their position during normal operation of the reactor and in case of an incident.
In the upper part, they often comprise a tube along which transverse plates are spaced apart for guiding the rods individually and, in the lower part, split tubes each assigned to a rod.
Cluster guides are also known in which the tube is replaced by vertical longitudinal members.
The guides are individually provided with a device for guiding and centering their lower part on the upper core plate. Different devices for guiding and centering the guide with respect to the lower plate of the internal equipments are known. They are generally designed so that the guides retain the possibility of moving axially, in particular to allow their free expansion in service and to facilitate mounting or removal thereof, while closing the guidance clearances so that the guides do not vibrate under the effect of the turbulence generated by the cooling fluid and limiting the possibilities of lateral deflection of the guides in the case of an earthquake or accidental depressurization of the reactor, due to the breakage of a pipe collecting the cooling fluid leaving the core; this allows the guides to remain in correct alignment with respect to the fuel assemblies and allows the control clusters to move freely.
The guiding devices are often formed of resilient members which have the drawback of fulfilling all the preceding functions simultaneously. Their design then proves difficult, for they must simultaneously be sufficiently flexible to accommodate, on fitting, the misalignments resulting from manufacturing tolerances on the guides and on the lower and upper plates of the upper internals and sufficiently rigid to prevent vibration of the guides, without excessive stresses in the resilient members during fitting.
French Patent No. 2,591,018 describes a cluster guide which, for separating the centering function from the function consisting in limiting the amount of lateral deflection, comprises a guiding and centering device formed by a set of several rigid blades having a radial clearance in an opening of the lower plate and by a set of a plurality of flexible blades having a shape at rest such that they bear resiliently on the edge of the opening in the lower plate. This approach is an advance over the prior art, but it does not completely separate the centering function from the vibration limitation function, since the flexible blade abutment participates in both.